Electrocatalytic fluoroalkylation of olefins

Article abstract of DOI:10.1007/s11172-010-0333-7

Under electrocatalysis conditions, the perfluoroalkylation of α-methylstyrene by the nickel complex NiBr₂bipy proceeds with the formation of dimeric products of addition of perfluoroalkyl radicals to the double bond.

Full text of DOI:10.1007/s11172-010-0333-7

Russian Chemical Bulletin, International Edition, Vol. 59, No. 10, pp. 1918—1920, October, 2010
1918
Electrocatalytic fluoroalkylation of olefins
D. Yu. Mikhaylov, T. V. Gryaznova, Yu. G. Budnikova, and O. G. Sinyashin
A. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Scientific Center of the Russian Academy of Sciences,
8 ul. Arbuzova, 420088 Kazan, Russian Federation.
Eꢀmail: tatyanag@iopc.ru, polyvoks@inbox.ru
Under electrocatalysis conditions, the perfluoroalkylation of αꢀmethylstyrene by the nickel
complex NiBr2bipy proceeds with the formation of dimeric products of addition of perfluoroalkyl
radicals to the double bond.
Key words: electrocatalysis, nickel bromide complex with bipyridine, electrolysis, perꢀ
fluoroalkylation, polyfluoroalkyl halides, αꢀmethylstyrene.
Organofluorine compounds possess unique physical
of olefins16 and the organic nickel σꢀcomplexes with the
Ni—C bonds are the key intermediates of these processes.17
The combined electrochemical reduction of NiBr2bipy and
polyfluorinated alkyl halides (RFX, Х = I, Br) in the presꢀ
ence of αꢀmethylstyrene affords the product of olefin
polyfluoroalkylation. Electrolyses were carried out both in
a divided electrolyzer at the Pt cathode and without diviꢀ
sion of the cathodic and anodic spaces at the Pt cathode
and using the Mg anode. During the reaction, the reaction
mixture turns dark claretꢀcolored when passing electricꢀ
ity. This color is characteristic of organic nickel σꢀcomꢀ
plexes with the Ni—C bond. As a result, the final products
were isolated as white crystalline compounds. Analysis of
the conditions of electrolyses and subsequent isolation of
the addition products showed that the electrosynthesis of
the fluoroalkylation products of αꢀmethylstyrene in a diꢀ
vided electrolyzer is most efficient.
The obtained addition products, viz., 2,3ꢀdimethylꢀ
1,4ꢀbis(perfluoroalkyl)ꢀ2,3ꢀdiphenylbutanes 1 and 2
(Scheme 1) were characterized by 1Н NMR spectroscopy,
IR spectroscopy, Xꢀray diffraction analysis, and mass
spectrometry. Analysis of the spectral characteristics of
the compounds obtained made it possible to assign them
the dimeric structure of the addition products of polyꢀ
fluoroalkyl halides to αꢀmethylstyrene. It was also noticed
that greenishꢀblue crystals precipitated from the reaction
mixture after standing for 3 days. According to the specꢀ
tral and cyclic voltammetry data, the crystals corresponded
to the starting complex NiBr2bipy.
and biological properties, due to which they are widely
used in medicine and agricultural chemistry and as various
materials.1—3 However, a few such compounds were found
in nature.4 Synthetic compounds compose the most part
of organofluorine products. There are different methods
for introducing fluoroalkyl groups into substrates. Howꢀ
ever, these methods are often poorly selective and have
a series of drawbacks (high temperature of synthesis, low
yields of final products). It is known that the addition
of perfluoroalkyl groups to olefins and acetylenes is
one of the most important methods of introduction of
perfluoroalkyl groups at the carbon atom.5,6 Freeꢀradical
reactions of addition of polyfluoroalkyl iodides to unsatꢀ
urated compounds are initiated, as a rule, by high temꢀ
peratures, ultraviolet irradiation, peroxides, metals, and
metal complexes.7—9 Sulfinatodehalogenating agents, such
as dithionites, sodium disulfite, and several others, are
often used as radical initiators.10,11 The electrochemical
generation of fluoroalkyl radicals is successfully perꢀ
formed.12—14 Recent success of electrocatalytic reacꢀ
tions allows one to hope that the use of electrochemical
methods of fluoroalkylation of various substrates would
contribute to the intensive development of this field of
chemistry.15
Results and Discussion
With the purpose to search for new routes of electroꢀ
chemical polyfluoroalkylation of olefins, the electroꢀ
catalytic addition of polyfluoroalkyl halides to αꢀmethylꢀ
styrene and vinyl acetate in the presence of the organoꢀ
nickel catalysts was studied. No data on styrene are preꢀ
sented because the polymerization product is mainly obꢀ
tained by electrolysis in this case.
Electrolysis of 6Hꢀperfluorohexyl iodide and vinyl acetate
in the presence of catalytic amounts of NiBr2bipy proceeds
similarly to form the dimeric addition product, namely,
2,3ꢀdiacetoxyꢀ1,4ꢀbis(6Hꢀperfluorohexyl)butane (3).
It is known that the cyclic voltammograms of the unꢀ
saturated complex NiBr2bipy contain two reversible reꢀ
duction peaks.18 At the first peak, NiBr2bipy undergoes
reversible twoꢀelectron reduction to the highly reactive
It is known that the nickel complexes are efficient cataꢀ
lysts of reactions of crossꢀcoupling and functionalization
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1868—1871, October, 2010.
1066ꢀ5285/10/5910ꢀ1918 © 2010 Springer Science+Business Media, Inc.

Electrocatalytic fluoroalkylation of olefins
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 10, October, 2010
1919
Scheme 1
Scheme 3
RF = C6F13 (1), H(CF2)6 (2)
X = I, Br
complex Ni0bipy (Scheme 2). Upon the addition of
polyfluorinated alkyl halide, in particular, perfluorohexyl
iodide С6F13I, to this solution, the cathodic current of the
first wave increases and reversibility disappears. Based on
this fact, we may assume that the complex Ni0bipy rapidly
reacts with the substrate in the oxidative addition reaction
to form an unstable σꢀcomplex C6F13NiIbpy, which probꢀ
ably decomposes easily to the polyfluoroalkyl radical and
the starting nickel complex through reductive eliminaꢀ
tion. It is this process which increases the cathodic curꢀ
rent of the reduction wave NiII/Ni0.
cycle and to model particular steps by the combined use
of spectroscopic and electrochemical methods will be
made further.
The reduction of RFX at the electrode is a oneꢀelectron
process and occurs at more positive potentials (–1.7 V
vs Ag/AgNO3) than the reduction of olefin. However, in
the absence of a nickel catalyst, their combined electroꢀ
chemical reduction affords no products of addition of the
RF moiety to the double bond.
In the present work, the use of the electrocatalytic
approach allowed us for the first time to carry out olefin
perfluoroalkylation under mild conditions in 50—70%
yield, and the moiety with the long fluoroalkyl radical chain
was introduced. Interestingly, the firstly obtained product
is not a product of the simple addition of the RF moiety to
olefin, but the dimer of the intermediate adduct, namely,
RFꢀolefin. Thus, the new scheme of olefin functionalizaꢀ
tion, which was not described earlier, was accomplished.
Scheme 2
NiBr2bipy + 2e
Ni0bipy + RFX
Ni0bipy
RFNiXbipy
RFNiXbipy
RF• + NiX2bipy
It should be noted that the organic nickel σꢀcomplexes
with the Ni—C bonds can add to some olefins in reductive
elimination reactions.18
Based on the results obtained, we propose the scheme
of reactions (Scheme 3) including the steps of oxidative
addition and reductive elimination.
Experimental
The nickel complex in the low oxidation state Ni0bipy
is generated during the electrochemical reduction of the
complex NiBr2bipy. If the reaction mixture contains
polyfluoroalkyl halide (С6F13I, С6F13Br, С6HF12Br),
oxidative addition affords the σꢀcomplex RFNiХbipy
(Х = Br, I), which accepts an electron during electrolysis
and transforms into the active complex RFNiIbipy capable
of adding to olefin due to radical decomposition. The comꢀ
plexes formed are, most likely, very unstable and undergo
further transformations into the final dimeric addiꢀ
tion products. In addition, the starting nickel complex
NiBr2bipy is regenerated as a result of this reductive elimiꢀ
nation. It was shown for the numerous electrocatalytic
reactions involving the NiBr2L complexes and organic
halides and studied in detail that the completion step of
reductive elimination resulting in the final dimeric prodꢀ
uct of other crossꢀcoupling product is always nickelꢀcenꢀ
tered.16 Attempts to isolate intermediates of the catalytic
Preparative electrolyses were carried out using a B5ꢀ49 dc
source at a current strength of 100 mA h–1 in 100ꢀmL threeꢀ
electrode cell. The potential of the working electrode was deꢀ
tected by a V7ꢀ27 dc voltmeter. The working surface of the
platinum cylindrical cathode used as a working electrode was
20.0 cm2. A ceramic plate with the pore size 900 nm was used as
a membrane. A platinum grid served as an anode, and the anolyte
was a saturated solution of the background used in the catholyte
in the corresponding solvent.
1H NMR spectra were recorded on a Bruker Avance DRX
400 spectrometer (400 MHz) relative to the signal of residual
protons of the deuterated solvent used as an internal standard.
IR spectra were measured on a Vectorꢀ22 spectrometer (Bruker)
in the frequency range from 400 to 3600 cm–1 in KBr pellets.
Electron ionization (EI) spectra were obtained on a TRACE
MS quadrupole mass spectrometer (ThermoQuest). A probe was
supplied using the direct injection process (DIP) with water
cooling. Peaks of fragmentation ions with the relative intensity
less than 5% are not presented.

1920
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 10, October, 2010
Mikhaylov et al.
Dimethylformamide was dried with K2CO3 and CaH2 and
distilled from molecular sieves. Benzene was dehydrated by
distillation with sodium. Purified anhydrous solvents were stored
under a dry argon atmosphere.
The supporting salt Et4NBF4 was prepared by mixing an
aqueous solution of Et4NOH (30%) and HBF4 until the neutral
pH value of the indicator. The precipitate formed (Et4NBF4)
was filtered off, recrystallized twice from ethanol, and dried in
a vacuum desiccator at 100 °С for 48 h.
Fluorinated alkyl halides (Close Corporation NPO P&M
Invest) and αꢀmethylstyrene (Acros) were used. αꢀMethylstyrene
was purified by fractionation. All syntheses were carried out unꢀ
der a dry argon atmosphere.
liquid. 1Н NMR ((CD3)2CO), δ: 2.03 (s, 3 H, CH3); 2.12 (s, 3 H,
CH3); 2.63 (m, 2 H, CH2); 2.80 (m, 2 H, CH2); 4.39 (2 Н, СН);
2
3
6.81 (tt, 2 Н, JН,F = 50.86 Hz, JН,F = 4.51 Hz). IR (KBr), ν/
cm–1: 1142, 1207, 1235 (C—F); 1760 (C=О). MS (EI), m/z (Irel
(%)): 773.93 [M]+ (80). Found (%): C, 30.39; H, 1.35.
C20H14F24O4. Calculated (%): C, 31.01; H, 1.81.
Xꢀray diffraction analyses of compounds 1 and 2 were carried
out at the Department of Xꢀray Diffraction Studies (Collective
Use Center of the Russian Foundation for Basic Research) on
a Bruker Smart Apex II diffractometer at 20 °C (MoꢀKα radiaꢀ
tion, multiscan mode). The structures were solved by a direct
method using the SIR program and refined first in the isotroꢀ
pic approximation and then in the anisotropic approximation
(SHELXLꢀ97 and WinGX programs). Positions of hydrogen
atoms were revealed from difference series of the electron denꢀ
sity and refined in the isotropic approximation.
Synthesis of 2,3ꢀdimethylꢀ1,4ꢀbis(perfluoroalkyl)ꢀ2,3ꢀdiꢀ
phenylbutanes 1 and 2 (general procedure). The complex NiBr2bipy
(1.5 mmol), perfluoroalkyl halide (15 mmol), and αꢀmethylꢀ
styrene (15 mmol) in DMF (70 mL) were placed in an electroꢀ
chemical cell. Electrolysis was carried out with divided anodic
and cathodic spaces with magnetic stirring at a continuous argon
flow. Electricity (1000 mA h–1) was passed through the electroꢀ
lyte. After the end of electrolysis, the reaction mixture was washed
with water and the organics was extracted with benzene (3×100 mL).
The organic layer was dried with MgSO4 for 24 h, the solvent was
concentrated, and the white crystals formed were filtered off,
washed with diethyl ether, and dried in vacuo with an oil pump.
2,3ꢀDimethylꢀ1,4ꢀbis(perfluorohexyl)ꢀ2,3ꢀdiphenylbutane (1).
The yield was 4.6 g (70%), m.p. 160—162 °С. 1Н NMR (C6D6),
δ: 1.46 (d, 3 H, CH3, 3JН,Н = 3.3 Hz); 2.25, 3.23 (both m, 2 H,
CH2); 7.07—7.19 (m, 5 H, C6H5). IR (KBr), ν/cm–1: 1144,
1208, 1237 (C—F); 1602 (C=C, arom.); 3069 (НС=). MS (EI),
m/z (Irel (%)): 437.0 [1/2 M]+ (100), 418.0 [1/2 M – F]+ (5),
The Xꢀray diffraction data for compounds 1 and 2 were deꢀ
posited with the Cambridge Crystallographic Data Centre
(CCDC Nos 720 933 and 720 934, respectively) and are availꢀ
able at www.ccdc.cam.ac.uk/data_request/cif.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 10ꢀ03ꢀ00335ꢀa).
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167.0 [1/2 M – C5F11 – H]+ (20), 149.1 [1/2 M – C5F12 + (18),
]
+
123.0 [1/2 M – C6H5 – C4F10 – H]+ (42), 118.0 [1/2 M – C6F13
]
(36), 103 [1/2 M – C6F13 – CH3]+ (40), 91.0 [C7H7]+ (60), 69.0
[CF3]+ (20). Found (%): C, 41.28; H, 2.45. C30H20F26. Calcuꢀ
lated (%): C, 41.19; H, 2.29.
2,3ꢀDimethylꢀ1,4ꢀbis(6Hꢀperfluorohexyl)ꢀ2,3ꢀdiphenylꢀ
butane (2). The yield was 3.2 g (49.6%), m.p. 156—158 °С.
1Н NMR (C6D6), δ: 1.36 (d, 3 H, CH3, 3JН,2Н = 3.5 Hz); 2.22,
3.15 (both m, 2 H, CH2); 6.063 (tt, 1 Н, JН,F = 52.09 Hz,
3JН,F = 5.14 Hz); 7.00—7.20 (m, 5 H, C6H5). IR (KBr), ν/cm–1
:
1145, 1208, 1238 (C—F); 1600 (C=C, arom.); 3071 (НС=).
MS (EI), m/z (Irel (%)): 419.0 [1/2 M]+ (30), 418.0 [1/2 M – H]+
(5), 167.0 [1/2 M – C5F10 – H]+ (15), 148.0 [1/2 M – C5F12 – H]+
(15), 123.0 [1/2 M – C6H5 – C4F10 – H]+ (35), 118.0 [1/2 M –
– C6F12 – H]+ (15), 103 [1/2 M – C6F12 – CH3]+ (35), 91.0
[C7H7]+ (60), 69.0 [CF3]+ (20), 51 [CF2H]+ (100). Found (%):
C, 42.58; H, 2.51. C30H22F24. Calculated (%): C, 42.96; H, 2.62.
2,3ꢀDiacetoxyꢀ1,4ꢀbis(6Hꢀperfluorohexyl)butane (3). The
complex NiBr2bipy (0.209 g, 0.56 mmol), 6Hꢀperfluorohexyl
bromide (2.39 g, 5.6 mmol), and vinyl acetate (0.48 g, 5.6 mmol)
in DMF (70 mL) were placed in an electrochemical cell. Elecꢀ
trolysis was carried out without division of the anodic and caꢀ
thodic spaces with magnetic stirring at a continuous argon flow.
Electricity (2 A per mole of the starting 6Hꢀperfluorohexyl broꢀ
mide (300 mA h)) was passed through the electrolyte. After the
end of electrolysis, the reaction mixture was washed with water
and the organics was extracted with benzene (3×100 mL). The
organic layer was dried with MgSO4 for 24 h, and the solvent was
concentrated. The residue was purified by chromatography.
Compound 3 was obtained in a yield of 0.98 g (51%), colorless
Received July 20, 2009;
in revised form June 22, 2010